So-called wall film effects occur in internal combustion engines having intake manifold injection. The fuel, which is injected into the intake manifold, does not completely enter the combustion chamber of the engine, rather, the injected fuel partially deposits as a wall film on the intake manifold or on the injection valves. This wall film mass changes during dynamic motor operation, especially, when there are changes of load. This leads to deviations from a pregiven air/fuel mixture ratio in an exhaust-gas system of the engine. The change of the wall film mass is corrected via a model. In this way, the deviations from the pregiven air/fuel ratio in the exhaust-gas system can be compensated.